Evaporation apparatuses for evaporation of liquids, for making these more highly concentrated or for separating solid substances from solvents, are generally known and are used generally within the chemical industry. In the pulping industry, such evaporators are used in particular for evaporating the spent liquor from pulp cooking, which spent liquor has to be concentrated by removal of water so that it can be combusted in a later stage, for example in a recovery boiler. These evaporation apparatuses consist principally of a vertical plate cylinder with vertically disposed tubes through which the liquid intended for evaporation is made to flow, while steam at higher temperature and pressure is made to flow in the opposite direction and condense on the outside of the tubes. The necessary heat for evaporation of the liquid, in the case of spent liquor from the pulping industry, is thus supplied through the tube walls. It is customary for the liquid intended for evaporation to be made to run downwards and for the heating steam to flow upwards in separate systems. The evaporated liquid thus falls down into the bottom of the cylinder-shaped evaporation apparatus, which bottom is usually slightly widened, and the vaporized steam is removed at the bottom part slightly above the pool of liquid which is formed in the preferably basin-shaped bottom part by evaporated liquid.
There are many different constructions of such evaporation apparatuses. Thus, evaporators can be arranged for motive steam to flow through the inside of the tubes while the liquid to be evaporated is made to run down the outside. Horizontally disposed tubes may also be provided, as well as plates whose surfaces are made to serve as evaporation surfaces.
In the pulping industry it is customary to evaporate black liquor in several stages, by series or parallel coupling of several evaporation apparatuses. The units, which are called effects, are numbered in accordance with the steam supply in the installation. Thus, fresh steam is supplied to the first effect, and in the subsequent stages the liquor steam which has been obtained in preceding stages is used as a heat source. This is possible by means of the successive reduction in pressure. Liquor steam from the last effect condenses in one or more surface condensers. By allowing the liquor steams to condense out in stages on different heat surfaces in the installation, it is possible to separate off, on the one hand, highly contaminated condensate for purification in, for example, a stripper column, and, on the other hand, purer condensate which can be used directly in the mill without odour treatment or other purification.
At the present time, ever greater demands are being made in most countries for a cleaner environment. Water which is to be discharged into rivers and the like must be pure, or at least so pure that it does not cause environmental problems. It is no longer possible to discharge semi-pure condensate. One way of reducing discharges could be for the condensate to be re-used as process water in the mill.
Insufficiently careful separation of evaporated liquid from vaporized steam, by means of small or large droplets being entrained with the steam, so-called liquor carry-over, which gives the condensate formed a brownish character, means that the condensate cannot be returned without further cleaning to the fibre line, e.g. for bleaching. The separation of the evaporated liquid from vaporized steam must therefore be virtually complete if the condensate is to be re-used as process water in the last stages of the fibre line, without impairing the quality of the final product, the paper pulp.
According to the prior art, a vertical cylindrical space without a bottom has therefore been arranged under the evaporation surfaces, by means of which the steam and the droplets are made to flow downwards, the larger droplets settling on the underlying bottom of the evaporation apparatus, and the small droplets being carried along in the flow of steam which is deflected upwards and separated in a droplet separator for vertical approach flow. After this separation of the small droplets which come together in the droplet separator and run down into the pool of liquid, the purified steam is led out from the effect.
Since the steam flowing upwards on the outside of the cylindrical space will have different speeds in different parts of the annular space outside the skirt, the droplet separation will also be non-uniform. The speed of the steam will be greatest nearest the outlet opening, and at the diametrically opposite side of this it is possible to have very low speeds and even downward flows through the droplet separator. There is therefore a considerable need for an even approach flow of vaporized steam towards the droplet separator and a consequently better separation of the droplets, with concomitant reduction in the cost of the construction.
According to the present invention, the above-mentioned problem of carry-over of liquid droplets has therefore been solved by making available an evaporation apparatus for evaporation of liquids, such as spent liquor from pulp cooking, comprising a vertical cylinder with inlet and outlet openings for steam and liquid, and evaporation surfaces in the cylinder, the apparatus being arranged to generate a downward flow of evaporated liquid and driven-off steam, which is characterized in that the outlet opening for driven-off steam is situated in the lower and preferably widened part of the cylinder, and in that the lower part of the space under the evaporation surfaces, the "skirt", surrounding the evaporated liquid and driven-off steam, is cylindrical with a smaller cross-sectional diameter than the lower part of the cylinder which has the outlet opening for steam, and in that the skirt, in addition to having an open bottom, also has an opening for outflowing steam on that part of the skirt which faces away from the outlet opening in the outer cylinder.
According to the invention, it is expedient for the opening in the skirt to be made by means of the latter having been cut obliquely downwards.
It is however possible, according to the invention, for the opening in the skirt to be made by means of its having been cut in a stepped shape downwards.
According to the invention, the skirt, with its lower edge not cut away, is intended to extend down into the underlying pool of evaporated liquid in the bottom part of the cylinder.
According to the invention, it is expedient for the skirt to be curved inwards at its upper part.
According to the invention, droplet separators for horizontal approach flow of steam are expediently arranged, in the direction of movement of the steam, in front and on both sides of the outlet opening.
According to the invention, the droplet separators can consist of two or more sections arranged one above the other.
According to the invention, the upper section or sections of the droplet separators are expediently arranged, in the approach flow direction of the steam, in front of the underlying section or sections.
According to the invention, drainage systems for liquid emptying into the pool of liquid in the bottom part of the cylinder are expediently arranged in each section of the droplet separator.